Aircraft altimeter



C. B. TERRY AIRCRAFT ALTIMETER Fi led Jan. 18, 1952 Sept. 29, 1936.

5 sheets-Sheet 1 MIL recs/wee INVENTOR CLIFFORD axearw #Q Z ATTORNEY u mm D m u w W U u A m Sept. 29, 1936. c, B. TERRY AIRCRAFT ALTIMETER FiledJan. 18, 1952 5 Sheets-Sheet 2 INVENTOR CLIFFO B. RRY BY ATTORNEY Sept.29, 1936. 2,055,883

C. B. TERRY AIRCRAFT ALTIMETER Filed Jan. 18, 1932 5 Sheets-$heet 3 dizy 5 j a i warm 3 Wrrse 5 9 A 15 1/ 7 macs/v52 INVENTOR CLIFFORD a, TERRYATTORN EY was awu'zaw wu HM;

C. B. TERRY AIRCRAF'JL ALTIMETER Sept. 29, 1936.

Filed Jan. 18, 1932 5 Sheets-Sheet 4 IHIENTOR FY20? B.TERRY ATTORNEY VIB w n Ll H w Sept. 29, 1936. c, B. TERRY AIRCRAFT ALTIMETER Filed Jan.18, 1952 5 Sheets-Sheet 5 INVENTOR CLIFFORD B TERR ATTQRNEY PatentedSept 29, l 936 gATENT OFFICE aosases Amcam ALTIMETER Clifford B. Terry,East' Orange, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application January 18, .1932, Serial No.587,264

20 Claims.

This invention relates to means for measuring distance and more inparticular to meansto be used on a plane for measuring the distancebetween the plane and ground, or between the "-.plane and a mountain orother obstruction in the path thereof. For purposes of descriptionapplicant has indicated thatthe measuring 'means'is to be used on aplane. Applicant does not intend to be limited by such description l0since, obviously, the measuring means may be used on any mobile craft,vessel or vehicle. For purposes of illustration it will be assumed:thatthe means of the present invention is located on a plane.

i It has heretofore been the practice when measuring the altitude of aplane from the ground to utilize an aneroid barometer calibrated interms of height of the plane above ground.

This barometer is installed on the plane and as the plane ascends theair pressure on the barometer decreases. This gives a fairly goodindication of height to within a hundred feet or so when the planecarrying the barometer starts from the same height above sea level eachtime.

However, when ascending or descending at the ordinary rate there isfound to be a lag in the barometer reading of about 200 feet or more.This renders th'e b'arometerpractically useless for altitude measuringwork close to the ground.

In the devices used/heretofore the scale of the altimeter must besetbefore the plane leaves the ground to a point corresponding with theair pressure at the point and time of departure.

The air pressure varies with time and locality so that. assuming a pilotto have flown from one field and set his altimeter there, it is notlikely that the altimeter will be correct when he lands at a differentfield at a different altitude some distance away some time later.Furthermore.

the altimeter will need resetting before the plane ascends again orsubsequent readings will be inaccurate.

Another serious disadvantage attending the use of altimeters usingpressure to denote the height lies in the fact that they do. notindicate to the pilot the distance 'of the ground in the direction inwhich the plane is flying. Thus a plane might readily fly into amountainside or other object in cloudy or rainy weather.

Briefly and broadly, the object of applicants invention is to provide adistance measuring device which will serve as an altimeter and whichwill overcome all of the disadvantages enumerated above.

' More in detail. an object of applicant's invention is to provide adistance measuring device by means of which the height of aplane fromearth or the distance from a plane to an object in its course may beestimated correctly and quickly at all times and which will give cor- 5rect readings even when the plane is only a fewfeet above the earth,or"from the objecttoward which itis flying. v

Another object of applicants invention is to provide an altimeter whichneeds no adjustment 10 after it is installed and which will, givecorrectreadings no matter what the height above sea level, from whichtheplane starts the flight.

A further object of the present invention is to provide an altimeter ordistance indicating 15 device, as set forth above, which will not onlyindicate to the pilot the distance the planeis above earth but will alsoindicate to the pilot the distance between the plane and any object inthe planes path, as, for instance, a mountain. 20 building, etc.

The above objects are attained in one instance in accordance with thepresent invention by means of a radio frequency signalling systemwherein a high frequency is generated intermittently and-sends outimpulses from a transmitter, which impulses strike the earth or anobjectin the path of the plane from which they are' reflected back to areceiver. The receiver is operative only when no impulse is sent by thetransmitter. An indicator cooperating with the receiver and transmitteris actuated only by said reflected wave to indicate 'the time the waveused in travelling from the transmitter to the earth or other'object andreturning to the receiver. This time will be equal to the distancebetween the plane and the object divided by one half the speed of light.

More in detail, distances are measured in accordance with onemodification of applicant's invention by producing at a transmitter avery I high frequency keyed by a. second oscillator at a considerablylower frequency so that only a few cycles of "high frequency energy at atime are radiated. I The transmitter at the same time charges acondenser which has crossing its terminals a suitable resistance. This:resistance, when the transmitter is on space, discharges the condenserat a predetermined rate. A receiver adjacent the transmitter is providedwith means 50 controlled by the low frequency oscillator for renderingit inoperative when the transmitter is operative thereby preventing thetransmitted energy from directly affecting the receiver. The receiverrendered operative by the reflected wave 5 flected from the ground orother object and reaches. the receiver the condenser will have a chargedependent on the length of time between the cutting of! of thetransmitter and the reception of the reflected wave at the receiver. Theaverage value of the charge on this condenser may be utilized in amanner which will appear more in'detail hereinafter to indicate thedistance between the reflecting object and the receiver. I

A modification includes'a plurality of sets of condensers andresistances of different sizes. These condensers and resistances beingofdifferent sizes have different time constants, thereby giving thealtimeter a different range over the scale of the indicator.

In a further modification the energy is fed from the intermittentlyoperative transmitter to the grid of a thermionic tube having-its anode'circuit connected in series'with the anode circult of asecondthermionic tube having its grid connected to the receiver, the tubesbeing biased to such a point that current can flow in the anode circuitonly when reflected energy appears in the receiver simultaneously withthe appearance of generatedenergy in the receiver directly from ,thetransmitter. A meter in the series anode circuit will indicate the timeof overlapping, of direct energy and reflected energy. This meter may becalibrated to indicate the height of the plane. This modificationdiffers from the prior modification as indicated and further, -in thatthe low frequency oscillator does notcontr'ol the operativeness of thereceiver. Variable. means is provided for compensating the directtransmission from the transmitter to" the receiving aerial so that thecompensating energy will reach the receiver in the proper phase andamplitude.

In a modification of this arrangement a single tube having two grids mayreplace the two tubes having their anode circuits connected in series.

.A further modification involves the use of a transmitter working at a.veryhigh frequency, a

receiver tuned to that frequency and coupled to the transmitter in sucha way as to be able to sent from the transmitter to the receiver thatwhenthe transmitter sends out a signal the receiver is not affected bythe direct radiation of the signal. on the receiver aerial. The receiverresponds to the reflected wave and operates to cut of! the transmitter.Obviously, when the,

transmitter stops sending the reflected wave will terminate. This inturn cuts off .the receiver,

which is normally biased-to cut off. A modulaaltitude of the plane. orthe distance between the tion of thetransmitter is effected, whichmodulation will have a frequency dependent upon the plane and theobject. The lower the plane or the nearer the plane to the object, thehigherthe frequency of modulationof the transmitter. The

direct reading meter need only be calibrated in feet to'serve as analtimeter. .The modulating frequency, that is, the. frequency at whichthe transmitter is modulated by-.-thefrefflected wave thr ush r eiver;will equal I 2,o5 s,sss furtherdiseharges the condenser resistance unit.LL! Consequently, when a transmitted wave is re- C cycles per secondwhere H is the height of the plane and C the speed of light. Thus, at

feet altitude, thefrequency of the meter will be 3000 kilocycles and at1000 feet only i 246 kilocycles. It will be obvious to the reader thatwith this altimeter extremely accurate readings may be';;taken close tothe ground or object in the path of the plane.

, The novel features of the present invention will be pointed out in theclaims appended hereto. 'I'henature of the invention and the mode ofoperation thereof will be better understood from the followingdescription thereof. when read in connection with the drawingsthroughout which like reference characters indicate like parts, and inwhich Figures 1, 3, and 5 show different modifications of applicant'sdistance meters; while Figures la, 3a, and 5a show the circuit detailsof the modifications of Figures 1, 3, and 5.

Figures 2 and 4 show modifications of the arrangements disclosedinFigures 1 andv 3 respectively while;

Figures 2a and 4a show modifications of the arrangements of Figures 2and 4 respectively.

Referring to Figure 1 of the drawings, T indicates a high frequencytransmitter, while B indicates a receiver which may be tuned to thefrequency of the oscillations generated by the transmitter. Thetransmitter circuits per se, and the receiver circuits per se, will bedescribed more in detail hereinafter by reference to Figure 1a of thedrawings. A clearer understanding of the invention will probably be hadby first describing broadly the several units involved and theircooperation. The transmitter Tenergizes the transmltting aerial TAthrough a line I. A receiving aerial RA is connected .through the line 2to the receiver. An oscillator K, operating at a frequency which is lowcompared to the frequency of the transmitter, T, is coupled through line3 to the transmitter T in such a manner as to modulate the transmitterfrom maximum to zero each cycle of the oscillator K so that thetransmitter radiates'only once each cycle of the oscillator K. Thetransmitter accordingly radiates a series of dots or signal impulses ofhigh frequency energy the frequency of the dots or impulses being thefrequency of the oscillator K. These dots are kept within small timelimits so that only a few cycles of the high frequency oscillationsgenerated'by T are radiated to form each dot. oscillator K is connectedby line 4 to receiver R The-- to send ,over a low frequency controllingwave which is utilized in any well known manner torender the receiverinoperative while the transmitter is operating. When the transmitter ison space, that is, is not sending out dots, the receiver R is permittedto receive signals. The receiver R picks up the radiated wave as it isreflected from the ground or nearest object. The time be-' tween thestopping of the transmitter, that is, the time at which it is cut off byK, and the time at which the reflected wave is receivedon the receiver,.will be an indication of the distance between the transmitter and thereflecting object. The means for measuring this time will now bedescribed.

, Apair'oi tubes A and B are'connected to the transmitter and receiverrespectively as shown by lines ,5 and 1. Tube A has its controlelectrode 9 negativelybiased to out 01f by means of a battery 75 a,while tubeB has its control electrode n negacuit of both tubes A and B.The' conductivity of.

- the anode cathode impedance of tube A is conresistance I 0. In themeantime the receiver has trolled by the transmitter, through line 5.The

conductivity of'the anode cathode impedance of tube line, I. l

In operation, assume that no slgnalis being sent out by the transmitter.The grid of A being normally biased to cut off,-no current flows in theanode cathode circuit of A. The condenser resistance unit 12, I0 'isaccordingly not charged. Assume now that. a dot is sent out by thetransmitter. The dot is radiated and simultaneously a highpositive-potential is supplied to the control electrode of A .throughline 6, This high 'potential overcomes .the normal effect of biasingbattery 6 and allows A to pass anode cathode current Current flowing inresistance In charges the condenser 12 slowly. As soon as the dotceases, that is, the transmitter stops operating, due to 'modulation tocut off by K, current stops flowing through the anode cathode circuit ofA and the condenser i2 is no longer charged. The

charge in condenser l2 starts to leak off through been inactive, that isbiased to out oif by energy from K over line 4, while tube B has beennonconducting since normally biased to cut off by battery 8. The wavetransmitted by .the transmitting aerialTA, hasbeen sent out andreflectedback toa receiver.

ward the receiver aerial RA. A short time after the dot has left thetransmiter it is picked up on the'receiving aerial RA and passes throughline 2 to the receiver. The receiver responds to the dot and current.flows in the output. circuitof' the This current overcomes the negativebias of 8 and makes the control electrode ll of B sufficiently'positiveso that B becomes conductive and current flows in the anode cathodecircuit of'B. i The charge on the condenser I2 is discharged through theanode cathode circuit of B. 'In discharging through B the current flowsthrough meter M.- The charge remaining on the condenser ll when-thetransmitter was cut oif to be discharged through the meter M will dependuponv the length of time required for the trans- I mitted dot toreachthe reflecting object and return to the receiving aerial. Since thedots follow each other at a rapid rate the condenser [2 will bealternately charged and discharged at a frequency determined by thedistance the dots haveto travel and the rate at which 12 dischargesthrough the resistance i0. Obviously, the scale of the frequency meter Mcan be calibrated in terms of distance above the earth or away from thereflecting object. By the use of'high radio frequency signals and shortdots the device can be made extremely accurate and will give accurateindications close to the ground.

- Since the time constants of the condenser l2" a d resistance l0determine the scale reading of l t e meter M, in order to be able totake accurate ,'i'eadings over a largescale, applicant proposes toprovide as indicated in Figures 2 and-2a a plu- B is controlled bythe-receiver throughrality of sets of condenser resistance units In, l2;l0, l2 etc., which may be switched in as indicated by a switch l6cooperating with contacts II. In this manner any one of a number ofunits, each having difl'erent-time constants,

. may be connected in series with the meter M in the anode cathodecircuit of B. The single meter may'have a plurality of scales calibratedthereon, which scales cover a large range of frequencies.

The method of and means for generating high frequency oscillationsusedin transmitter T and for generating low frequency oscillations in K,the circuit arrangement of the receiver R, and

the means and method for overcoming. the bias normally applied to tube Bby energy derived from the receiver R on the reception of the re--flected wave, will now be described by referring 'to=Figure 1a of thedrawings.

In Figure la of the drawings the transmitter T, including the highfrequency oscillation generator and the low frequency oscillator K, is

known today, for purposes of illustration 2, pre- -ferred form oftransmitter oscillation generator and transmitter have been shown. Thetransmitter includes thermionic oscillator 20 having a tuned gridcircuit 22 and tuned plate tank circuit 24. The tuned plate tank circuit24 includes a tapped inductance connected through a blocking condenser23 to the control electrode 25 of a thermionic amplifier 26. Thethermionic amplifier 26 is of the screen grid. type and has its .anodecircuit connected through a coupling inductance 28, tuned by condenser21, and a choking inductance 29 to the positive terminal of a source ofpower. The source of potential is not shown but may be the same as thatby means of which the anode circuit of 20 is charged through ch'arginginductance 3|. High frequency oscillations are shunted around thissource by condenser C. The transmitting aerial Ta is connected to aninductance 30 coupled to inductance 28. High frequency oscillationsgenerated by 20 will be amplified by 26 and impressed on 30 .to beradiated from Ta. In order to control the transmitter so that only shortpulses of high frequency oscillations are radiated, the screen gridelectrode 31 of the amplifier 26 is connected with the low frequencyoscillator K in such a manner that a modulating potential at thefrequency of K is applied to the screengrid to alternately start andstop radiation from the output of the screen grid amplifier. The lowfrequency oscillator K includes a thermionic tube 32 having a gridincluding a frequency control piezo-electric crystal 33 shunted by aresistance R and a plate circuit including an inductance 34 tuned byparallel condenser 35; The inductance 34 is coupled to an inductance 36connected in series with the screen grid cathode impedance of theamplifier 26. The oscillations at a frequency determined by the crystal33 appearing in the circuit 36 will vary oscillator tube 32, modulationmay be obtained in the screen grid tube 26 by as much as 100 per cent,so that the transmitter radiates a series of the potential applied tothe screen grid electrode pulses separated by intervals during which noenergy is radiated. Radio frequency choke coils- I and 39 serve the samepurpose in the anode plied to the'input circuit or :tube A overcomes thenormal biasing potential applied to the grid 15 circuit of 52 and inthescreen grid electrode circuit of 25. Charging potential for thescreen grid electrode of 26 and-anode electrode of 32 is supplied from asource. not shown, although in circuit with the inductances 38 and 3.9,.v The inductance .28 inthe output circuit 01'. the transmitter is alsocoupled through a link circuit. 40

to the input circuit of the irequencymeasuring tube A. When a positiveimpulse is sent out by the transmitter the same positive impulseapelectrode thereof by 6 so that the tube 'A be-' Y comes conductive andthe condenser 12 in the output circuit thereof is charged during thetime the pulse is being sent out,:as set out inmore detail hereinbeiore.The receiver includes the receiving aerial Ra connected through'lines!to w an inductance 42 coupled to a tuned inductance 43 in the inputcircuit of thermionic tube 45 which operates to cut oil ,the receiver aswill appear hereinafter. The thermionic tube 45, which is of the screengrid electrode type, has an anode circuit including a tuned inductance44,

which is independently coupled. to-an inductance ductance 28 in theoutput circuit or the amplifier 25 in the transmitter. Potential for theanode circuit of D and M45 is supplied through a resistance 55 from thecommon potential source not shown. The potential applied through, 55

is such that when tube 50 is rendered conductive by an impulse appliedto the input circuit thereof through 5| the drain of anode currentthrough and 5,5 is such that the potential applied from 55 through 5| tothe anode electrode oi 45 is so low that 45 is nonconducting.-Accordingly, signals impressed on the'receiver R irom the transmitterwill not appear in the itnode circuit of the tube 45 unless they arereilected signals, since 45 is cut oil or rendered intjperative duringthe entire time a pulse is being transmitted. Signals appearing in theinput 49 cf 45 are repeated in the tuned output circuit 51 as directcurrent or unidirectional pulses, due to the rectifying action of tube45. These direct current pulses overcome the normal cut-ofipotentialapplied by biasing battery 8 tothe input elements of tube B sothat B becomes conductive. when B becomes conductive, as pointed outhereinbefore, the condenser l2 can discharge through the meter.

T Since the receiver is operative only when the transmitter isinoperative, the tube B becomes conductive only on the receipt of areflected signal. The action of the meter, due to current from thecondenser l2,- which is charged by A and discharged by B at -a rapidrate, will indicate the distance the signal has travelled from thetransmitter aerial to the reflector object and back.

The metering circuit, including the tubes A and B and the circuitsconnected between the elec- 2,o5.s,esa v trodes thereof, as shown inFigure 1a, may be replaced by the metering circuit including the tubes Aand B and associated circuit, as shown in Figure 2a. As the-operation ofa circuit as shown in Fig. 1a.modified as indicated in' 2a will beapparent 'from the description. of the former the operation of 2a willnot be set out in greater detail here.

In the prior described modiflcatiomindications Y of the meter arereceived only when the transmitterxis inoperative and the receiveractuated by a reflected wave overcomes the negative bias on the controlelectrode of B. In many cases it might be desirable to have the meteroperative during the time in which the transmitter is sending out a dotand the receiver is receiving the reflected dot, in other words,measuring the time in which the transmitted dot and the reflected dotoverlap. Such an arrangement isshown in 'the modification illustrated'inFigure 3. Here the transmitter T is keyed by low frequency oscillator Kbut the receiver is not rendered inoperative by K when T is operative.Direct radiation 'from the transmitter aerial TA reaching the receiveraerial RA is compensated by energy sent .over lineL from the transmitterto the receiver.

This line includes amplitude adjusting means I8 in the form of avariable coupling inductance and phase adjusting means in the form ofvariable taps i9 connecting the line L to the receiver. The points l9may be shifted along the transmission line L in a manner to pick ofienergy at the proper phase therefrom to compensate the radiated energyreaching the receiver RA from T by direct radiation. In this,arrangement as in the prior modification, the transmitter T is keyed ormodulated from zero to maximum amplitude by oscillations from theoscillator K,

Here, as in the prior arrangement, the transmitter feeds energy'to thenegatively biased control electrode 9 of tube A. Here, however, theanode cathode circuit of tube A and tube B is connected in seriesthrough the source M and meter M. Each of the tubes A and B are normallybiased negatively to cut oil by biasing batteries 6 and 8 as inFigure 1. v

In operation, assuming no signal is being sent out. The. control gridsof A and B, being negatively, biased to cut oil, prohibit current fromflowing in the anode cathode circuits. No current flows in the meter Mand no indication is registered. Assuming the transmitter T sends out a.dot, that is, a short series oscillations. The direct radiation from.the transmitter aerial on they receiver aerial is compensated by theline L so that the directly .radiated energy has no effect on thereceiver. 'The radiated wave reaches the reflecting object and returnsto the receiving aerial. RA. During this timethe transmittersends energyto the control electrode of tube A. This energy overcomes the negativebias applied by 6 to the control electrode of tube A. Normallythis-would allow anode cathode current to flow in tube A. Howsistance oftube A. When the reflected wave reaches the receiving aerial RA 2,positive poten tial is applied to the control electrode of tube B. Thispositive potential overcomes the negative of high frequency potentialappliedto the controlelectrode by '8 accuses thereon. This current.fiows through the meter M. Therefore, the meter will passcurrent-during the time in which the reflected wave isre- .ceived on thereceiver aerial RA and the transmitted signal is received on the grid ofA direct- 1y from the transmitter simultaneously. The length 01' timeduring which these signals are received simultaneously or overlap willbe representative oi the distance from the receiver aerial to thereflecting object. Accordingly, the meter is actuated only during thetime when the anode cathode impedance 01' both tubes A and B are lowereddue to the positive charge on the control ,grids thereon. In operationthe meter M will pass a pulse oicurrent at every, cycle or theoscillator K. The duration of this pulse will depend upon the distancebetween the' transmitting aerial and the reflecting object. Thefrequency meter M will assumes. position dependent upon the-altitude ofthe plane and the scale of the meterM may be calibrated in termsoialtitude.

The method and means for generatingthe'high frequency oscillations usedin. transmitter T and tor generating the .low frequency oscillations inK, the circuit arrangement 01' the receiver R, and

the means and method for operating the circuits.

associated with. the meter M, and for introducing energy directly fromthe. transmitter to the receiver circuit .to compensate energy radiateddi-' rectly Irom the transmitter aerial to the receiver V aerial, willbe understood by reference to Fig. 3a.

- In Figure 3athe oscillation generator and generator K and receiver Rare substantially similar to the lik'e'circuits of Fig. 1a; In thismodification,-however, the keying tube 50 for the receiver is notnecessary since inthis modification the receiver is not cut oil duringtransmission. However, in this modification compensating energy from thetransmitt'eris impressed on the input of the receiver by means 01' aninductance l8 variably coupled, 'as indicated at l8, with the inductance28 in the output circuit of the transinitter. Inductance l8 fee dsenergy to an inductance ll over the line L. The inductance .43" is, asindicated, variably connected to the line L by taps I9 so that the phaseof the high frequency oscillations reaching the secondary winding 43 intheinput circuit of the receiver amplifier 45 may be adjusted to opposeand compensate. the energy reaching the inductance 43 -i rom the aerialRA by direct radiation. Since in this modification the meter M is'notoperative until both tubes A and B are conducting, and

since A is conducting when the transmitter is operative, means-isprovided for rendering the tube B conductive when the reflected wavereaches the aerial. This means .takes the .i'orm of a line I inductivelyconnected on the one hand to the output circuit 51 of the rectifier 46of the receiver, and on the other hand to the input circuit of tube B.Unidirectional pulses in I, resulting from the rectified reflectedsignal'in the receiver, will overcome the bias normally applied to thegrid-of B through'battery 8 and render tube B conducting.

As pointed out in detail hereinbefore, when tubes A and B are bothconducting current will flow in the meter M." The length 01' theduration of the pulses flowing in tube M will be an indi-- cation of thedistance. of the receiver from the refleeting suriace. 1

As will be seen train the above, the basic prinwave. Obviously, it isnot necessary to confinethe measuring period to that period when thetransmitted and received wave overlaps, but 'instead, the, current maybe'allowed to flow only during the period when neither the transmitternor the receiver is operating. This may be accomplished by normallybiasing the tubes to' such a value that current may fiow through thetubes when the control grids thereof are not excited by signalfrequency. Signal frequency appearing on the control grids will overcomethis bias and bias the tubes to cut off rendering their impedances sohigh as to prevent current flowing through the meter. In this lattermodification the reading on the meter would decrease with increasingaltitude as in theprior modification since, as in the priormodification, the time during which the direct signal and the reflectedsignal overlaps, decreases with increase of altitude. The current mayalso be allowed to flow through the meter only when either thetransmitter or receiver are operating single. In this case the meterreading will increase on an increase in altitude.

A further modification as illustrated in Figs. 4 and 4a contemplates thereplacement of tubes A and B of Figs. 3 and 3a by a. single thermionictube A having a pair of 'control electrodes 20 and 22 and a single anode23. In this arrangement, as will be seen by reference to the drawings,the transmitter is connected directly through battery 6 to-the controlelectrode 20 oi the tube A, while the receiver is connected directlythrough the battery 8 to the control electrode 22 thereof. The

anode cathode circuit includes the source of potential l4 and the meterM in series. In this modification, as in the arrangement shown in Figs.3 and 3a, anode cathode current can flowonly'when a signal direct fromthe transmitter appears on the control electrode 201 at the same timethat energy representative of a reflected signal on the receiver aerialappears on the control grid 22 of A. This modification operates in amanner similar to the operation 01 the arrange-- fleeting object.This'device utilizes a transmitter T working at a very high frequency.-The transmitter T supplies energy to be radiated from transmitteraerial TA. A receiver R havinga receiving aerial RA is connected throughline L1 to transmitter T in such a manner that when. a signal isreceived on the receiving aerial. RA the receiver output circuit willkeythe transmitter in such a manner as to start and stop oscillationstherein intermittently as the reflected signal reaching the receivingaerial RA starts and stops. This is accomplished in accordance withthepresent invention by the use of a receiver substantially similar to thereceivers describedhereinbefore, and a transmitter similar to thetransmitters illustrated hereinbefore except that the modulating lfrequency generator K of the prior modifications is unnecessary in thismodification. Means is provided for connecting the output of thereceiver circuit to the final tube in the transmitter for keyingthe-transmitterby energy from the output or the receiver. In the lattermeans resides the main distinction between the present altitude meterand the altitude meters of the prior modifications. Here the receiveraerial RA is connected as in prior modifications through a screen gridamplifier 45 to a rectifier tube 48 by means of coupling inductances 44and 49. The unidirectional or pulsating current resulting in the outputcircuit of tube 46, due to rectifying action therein, is introduced intoa line L1 through coupling inductances 51 and .58. Line Ll. terminatesin an inductance 36' coupled to the inductance 36 in the screen gridcathode circuit of the amplifier 26 of the transmitter. The currentpulsations flowing in this line act through inluctance 36' andinductance 36 in the screen grid cathode circuit to modulate or key thetransmitter by modulating the constant direct current applied theretothereby actingon the conductivity of the amplifier 26 in the outputthereof. While in practice it is necessary to provide more amplificationthan is obtained by the single grid amplifier 45 for purposes ofsimplicity, a single ampliher for the received radio wave is shown priorto the detector or rectifier 46. It will be understood, however, thatfurther amplification may be provided before or after the rectifier 46withation from TA to the receiver aerial RA a line L is connectedbetween the transmitter and the receiver. This line L includes phaseshifting and. amplitude adjusting means PS in order that the energy fedtherethrough from the transmitter to the receiver may be of proper phaseand ampli- Cit tude to compensate the energy radiated from thetransmitter aerial directly to the receiver aerial.

In operation the frequency meter M indicates the frequency at which thetransmitter T is keyed by the receiver R. The transmitter is arranged tooperate continuously unless keyed by receiver R acting. through line L1.

Assuming the plane to be flying at a certain altitude and the transmit--ter to be sending out signals. The receiving antenna RA will pick uptwo signals, one directly from the transmitting antenna TA and the otherwhich reaches the aerial RA after being reflected from the ground orother object. The signal picked up directly from the transmitter aerialTA will be balanced out by fed to the receiver by line L, which has beenad.-. justed to transmit a signal to the receiver in the proper phaseand amplitude to compensate, the signal directly radiated thereto fromthe transmitter aerial. After the transmitted wave reaches the reflectedobject and returns to the receiver aerial unidirectional pulses flowingin the output circuit of the receiver act on the conductivity of theamplifier 28 through its screen grid circuit inductively coupled to theinductance terminating line-L. When the transmitter is shut off, thesignal acting on the receiver will cease a definite time later, the timedepending upon the time the signal has to travel to the reflectingobject and back to the receiving aerial. from the transmitter ceases andthe reflected signal ceases to reach the receiver aerial the means ofthe signal When the signal transmitter again operates and emits asignal.

This cycle, described above, will repeat itself indefinitely at afrequency dependent upon the distance between the transmitting aerialTA, the rerepeated will be representative of the height of the plane.The frequency at which the transmitter is modulated will be equal tocycles per second where H is the altitude and C is the speed of light.

The frequency meter M is arranged to measure directly the frequency ofmodulation, that is the rate at which the reflected signal shuts oil thetransmitter T and not the frequency of the transmitter itself. Thefrequency of this modulation will give an accurate indication of theheight of the plane above ground or the distance of the plane from thereflecting object. The scale used to measure this frequency may becalibrated in feet altitude instead of cycles. A frequency of 3000kilocycles per second will correspond to 80 feet altitude and frequencyof 246 lrilocycles per second will correspond to 1000 feet altitude.

I claim:

1. In a distance measuring device the combination of a high frequencythermionic transmitter, crystal controlled thermionic modulating meansfor said transmitter, a high frequency thermionic receiver, a controlcircuit connecting said receiver to said transmitter and adapted torender said receiver inactive when said transmitt'er is operative, animpulse measuring circuit including -a normally non-conductingthermionic tube connected with said transmitter, and a circuit forcoupling said receiver with said measuring circuit.

2. In altitude measuring means the combination of a high frequencytransmitter, a high frequency receiver, a keying circuit connecting saidreceiver to said transmitter and adapted to render said transmitterinactive when said .receiver'is operative, a thermionic relay and afrequency meter coupled with said transmitter and said receiver, saidthermionic relay being normally biased to cut oil, and means associatedwith said receiver and responsive to energy therein representative ofindirect or reflected energy fromsaid transmitter for overcoming thenormal bias on said thermionic relay.

3. In a distance measuring device the combination of a high frequencytransmitter, a high frequency receiver, an oscillator connected withsaidtransmitter and with said receiver and adapted to key said transmitterand to render said receiver inoperative when said transmitter is keyed,an indicating circuit comprising a pair of thermionic tubes having anodeand cathode circuits connected in parallel, a meter inone of saidcircuits, a time element device including a resistance and parallelcondenser in one of said circuits, means for normally biasing said tubesto cut ofl, means for overcoming the bias of one of said tubes includinga connection between a control electrode of one of said tubes and saidtransmitter, and means for overcoming the bias of the other of saidtubes including a connection between the receiver and the controlelectrodeof said last named tube.

4. In an altitude meter the combination of a high frequency transmitter,a thermionic'receiver,

a low frequency oscillator connected with said transmitter and with saidreceiver and adapted to key said transmitter and to render said receiverinoperative, an indicating circuit comprising a thermionic relayincludinga plurality of tubes with their. anode to cathode impedancesconnected in parallel, a meter in oneof said connections, 9. timeelement device including a resistance and parallel condenser in saidcircuit, means for normally biasing said tubes to cut 011, means forovercoming the normal bias of one of said tubes includinga connectionbetween a control electrode of one 01 said tubes and said transmitter,and means for overcoming the bias of another of said tubes includingaconnection between the electrodes of said last ing means including apair of thermionic tubes having anode and cathode electrodes connectedin series through a meter, and circuits for connecting the controlelectrode of one of said tubes to said transmitter, and thelcontrol'electrode of the other of said tubes to said receiver.

6. In an altimeter the combination of a therm'ionic transmitter, athermionic receiver, means for preventingthe direct radiation from saidtransmitter from afiecting said receiver, an electron discharge tubeconstituting a variable impedance connected with the output circuitofsaid transmitter, a" second electron discharge tube constituting avariable impedance connected with the output circuit of said 'receiver,the impedances of said discharge tubes being in series,

. and a meter in series withsaid impedances. I

7. In means for,measuring the distance from a transmitter to areflecting object, the combination of a transmitter and receiver, ameasuring ,.circuit comprising a pair of thermionic tubes, a

resistance and a condenser connectedbetween the output electrodes of oneof said tubes, means for connecting the output impedance of the othertube in parallel with said resistance and condenser, a meter in saidlast named connection,.

a coupling. between the input electrodes oi.one of said tubes and saidtransmitter and a coupling between the input electrodes oi. said othertube and said receiver.

8. An altitude indicating device including a transmitter, a receiverincluding an aerialresponsive to indirect radiation, thermionic keyingmeans for rendering said transmitter operative and inoperativeintermittently, means i'or rendering said receiver inoperative when saidtransmitter is operative, and means for indicatdng'the time it takes thetransmitted wave to reach the reflecting object andreturn to thereceiver aerial including, a thermionic tube having its input electrodescoupled to the output or said transmitter and its output electrodescoupled toa meter, and a second thermionic tube having its inputelectrodes coupled to said receiver and its output electrodes coupledtosaid meter.

9., An altitude indicating device including a transmitter, a receiverincluding an aerial in the vicinity of said transmitter, said receiverbeing responsive ,to' radiation which has been reflected from aconductive object only, automatic means for operating said transmitterintermittently, thermionic demodulating means connected between saidtransmitter and saidreceiver for rendering the latter inoperativeintermittently,

and thermionic means connected with the output 01' said transmitter andthe output of said receiver for measuring the time it takesthetransmitted wave to reach the reflecting object and return to I therece'lveraerial. A

10. In altitude measuring means the combination of an intermittentlyoperated high frequency transmitter, a high frequency transmitter, a control circuit connecting said receiver. to said receiver and adapted torender said receiver inactime when said transmitter is operative, a pairof thermionic tubes, an output circuit comprising a charging condenserand a resistance connected with the output electrodes of one of saidtubes, a I

source of potential in said circuit, means for connecting the impedancebetween the 'output electrodes of the other of said tubes inparallelwith the output circuit of the first named tube, a meter in saidlast named connection, and link circuit for connecting the inputelectrode of one of said tubes to the output circuit'of said trans--meter, a thermionic tube connecting said transmitter to said meter, athermionic tube connecting said meter to said receiver, means forpreventing direct radiation from said transmitter' from energizing saidreceiver, and means for energizing said meter-when indirect radiationreaches said receiver from said transmitter.

12, An altitude indicating device including a transmitter comprising, asource of'high irequen'cy oscillations, and athermicnic amplifier havingits input electrodes coupled to said'source of high'frequencyosciliationss'nd its output electrodes connected with an output circuit,receiving means including an aerial adjacent said transmitter, saidmeans being responsive to radiation Irom said transmitter which reachessaid aerial only after reflection from a conductive object, a thermionicamplifier in said receiving means connected with said receiver aerial, acircuit coupling said thermionic amplifier in said receiver to saidthermionic amplifier in said transmitter, and means including anindicator in said coupling circuit for indicating the time it takes thetransmitted wave to reach a reflecting objectand return to the receiveraeriaL,

13. In an altitude indicating device, a transmitter having a's'ource ofhigh frequency oscill'a- ,tions, an electron discharge tube amplifiercoupled to said source and having an output cir- '=cuit,-including atransmitting antenna, a receiver having aninput circuit, including areceiving antenna, and an electron discharge tube ampliher under controlof energy collected by said receivin g a'ntenna, means including amulti-grid electron tube for producing, modulations 01' said highfrequency oscillations in accordance with the operation of saidreceiver, means including I flected from a conductive object, andmeanscoupled to the output circuit of said transmitter to said receiverto reduce the other. of said imamplifler for indicating a. modulationcharacterfor producing and amplifying high frequency oscillations, athermionic receiver including signal absorption and signal amplifyingmeans, an oscillator connected with said transmitter to key saidtransmitter at the frequency of said oscillator, said oscillator beingcoupled by way of said transmitter amplifier to said receiver, anindicating circuit comprising a plurality of thermionic im, a circuitfor connecting said impedances in series by way of a meter, saidimpeda'nm being normally of such a high value as to prevent a flow ofcurrent therethrough, a circuit connected with said thermionic means andsaid transmitter amplifier to reduce one of said impedances when saidtransmitter is operative, and a circuit connecting said thermionic meanspedances when said receiver is operative.

15. In a distance measuring device the combination of, -a transmitterincluding a thermionic oscillation generator and a thermionic amplifierof the screen grid type' connected therewith, a thermionic receiver,said receiver including an absorption device and thermionic amplifyingmeans, means for preventing a direct radiation from said transmitterfrom acting on said'receiver including, a circuit having phase changingmeans coupled between the output circuit of said transmitter amplifiertube and the input circuit oi said receiver amplifier tube, a circuitcoupled between the output circuit of said receiver amplifier tube andthe screen grid electrode of said transmitter amplifier tube, and afrequency meter preventing direct radiation from said transmitter fromenergizing said receiver," and energizingmeans in a circuit common toboth of said thermionic impedances and said meter.

1']. An altitude indicating device including a areceiver including anaerial and demodulating means adjacent said transmitter,circuitspermitting energy resultingfrom indirect or reflected radiationonly to reach said demodulat ing means, automatic means for renderingsaid transmitter intermittently operative and inop-' erative, impulsecounting means and circuits coupling the output of the demodulatingmeansto said counting means and the output of said transmitter to saidcounting means.

18. In altitude measuring means, the combination of a high frequencytransmitter of a thermionic type, a high frequency receiver of thethermionic type, a control circuit connecting said receiver to saidtransmitter and energized by said receiver on the reception of energysent out by said transmitter and reflected to said receiver from aconductive object to render said transmitter inactive when said receiveris operative, a frequency meter connected with the output of saidtransmitter, and a circuit of variable impedance coupling the output ofsaid transmitter to the input of said receiver for preventing directradiation from said transmitter from energizing said receiver.

19. In a measuring device, the combination of a transmitter including athermionic tube, a receiver adjacent thereto, a circuit connecting saidreceiver to said transmitter for compensating direct radiation from saidtransmitter to said receiver whereby energy from said transmitteraffects said receiver only after reflection from a conducting object, aconnection between the output of said receiver and a control electrodein the thermionic tube of said transmitter, and a fre-- quency meterconnected with said transmitter output to indicate the frequency of theenergy in the output of said transmitter as controlled by said receiverby way of said connection between the output of said receiver and thecontrol. elecsaid transmitter, said receiver including an aeriallocatedadjacent said transmitter, a thermionic keying tube connected with saidtransmitter for means responsive to energy alternately delivered by saidtransmitter and by-said receiver for indicating the distance travelledby a radio wave emanating from said transmitter, reflected from aconductive object and then received by the aerial of said receiver.

CLIFFORD B. TERRY.

